An equalizer that has been used in a conventional receiver will be explained. In digital radio communications that are carried out using portable telephones or the like, a delayed wave that cannot be disregarded in data symbols based on a multi-path propagation could occur. When this delayed wave has occurred, interference is generated in data symbols. This is called inter-symbol interference (ISI). As one of reception techniques that overcome this ISI, there is equalization technique.
The operation of an equalizer that uses the above equalization technique will be explained. A replica preparation type maximum likelihood sequence estimator (MLSE equalizer) will be explained as one example (For the MLSE equalizer, refer to “Maximum-likelihood sequence estimation of digital sequences in the presence of intersymbol interference”, G. D. Forney, Jr., IEEE Trans. Inform, Theory, vol. IT-18, 3, pp. 363–378, May, 1972).
The MLSE equalizer estimates an amplitude, a phase, and a delay time of a signal that arrives with a time variance based on a multi-path propagation, by using known series called a unique word at a receiver side in advance. This estimates a channel distortion generated based on a multi-path propagation. A result of this estimation is called a channel impulse response (CIR). The MLSE equalizer prepares a reception signal candidate (a replica) for estimated data symbol candidates, by using this CIR. The MLSE equalizer compares this replica with a reception signal, decides a data symbol candidate corresponding to a replica that is most similar (a highest likelihood) as a decision data symbol, and carries out a demodulation. In this way, the MLSE equalizer compensates for a channel distortion due to a delayed wave by using the estimated CIR, thereby to conquer the ISI.
However, in the mobile communications, a signal channel changes to the other path depending on time when the terminal moves. In other words, the CIR that is necessary for the MLSE equalizer varies with time. The CIR estimated by the MLSE equalizer is the CIR that has been estimated at the time when a unique word has been transmitted. When a signal at a different time from this time is demodulated, a CIR variation becomes an error, and a characteristic is degraded as a result.
In the mean time, there has been proposed an adaptive equalizer that follows a variation in channels. (For the adaptive equalizer that follows a variation in channels, refer to “An adaptive maximum-likelihood sequence estimation for fast time-varying intersymbol interference channels”, H. Kubo, K. Murakami and T. Fujino, IEEE Trans. Commun., vol. COM-42, 2/3/4, pp. 1872–1880, February/March/April, 1997.) This adaptive equalizer estimates a CIR initial value by using a unique word. Further, this adaptive equalizer sequentially estimates CIRs that vary with time by using a demodulated data symbol, thereby to follow variations in channels. Based on this, it becomes possible to carry out the equalization processing even under a condition of fast changing channels.
A representative operation of an adaptive equalization apparatus will be explained below with reference to the drawings. FIG. 6 is a diagram which shows a structure of a conventional adaptive equalization apparatus. In FIG. 6, the reference number 1 denotes a receiving-signal input terminal, 2 denotes a decision value output terminal, 300 denotes a timing adjuster, 301 denotes a timing detector, 302 denotes an adaptive equalizer, and 303 denotes a CIR initial value setter.
In the conventional adaptive equalization apparatus shown in FIG. 6, the timing detector 301 first obtains a timing of a unique word based on a received signal, and the timing adjuster 300 adjusts the timing by using this timing. Next, the CIR initial value estimator 303 estimates a CIR initial value by using a unique word within a received signal after the timing adjustment. Finally, the adaptive equalizer 302 sequentially estimates CIRs that vary with time based on the estimated CIR initial value, and then obtains decision value by using the estimated CIR value and the received signal.
FIG. 7 is a diagram which shows a representative burst format when an adaptive equalization apparatus is used. As shown in the drawing, the burst format is constructed of a preamble, a unique word, and data.
FIG. 8 is a diagram which shows a communication system that comprises a receiver having the above adaptive equalization apparatus, and a base station. It is assumed that a mobile station 321 moves from one position (“position (A)”) to another position (“position (B)”) as shown in the drawing will be explained. The reference number 320 denotes a base station, and 321 denotes the same mobile station. The reference number 123 denotes a direct wave that directly reaches the mobile station 321 from the base station 320, and 124 denotes a wave from the base station 320 that is reflected by an obstacle and reaches the mobile station 321. The reference number 133 denotes a channel CIR in the mobile station 321, 134 denotes a channel CIR in the mobile station 321 after the movement, 143 denotes a set CIR in the mobile station 321, and 144 denotes a set CIR in the mobile station 321 after the movement.
The reflection wave 124 has to travel a longer distance than the direct wave 123, and therefore, arrival of this reflection wave 124 at the mobile station 321 is delayed. Assume that the arrival time of the reflection wave 124 is delayed by one data symbol component from the arrival time of the direct wave 123. The left-side CIR shows the direct wave and the right-side CIR shows the reflection wave.
When the mobile station 321 is at the position (A) during a reception of a unique word, and the mobile station 321 is at the position (B) at the time of finishing a burst (time required for the movement is assumed to be equivalent to one burst time), for example, the channel CIR 133 changes to the channel CIR 134. In this instance, the adaptive equalizer 302 estimates a CIR initial value by using a unique word, and obtains the set CIR 143 after that. At the data transmission time, the adaptive equalizer 302 updates the CIR, and follows CIRs finally up to the set CIR 144. A demodulation time is obtained from the unique word that is included in the direct wave 123, and a constant timing is obtained over the data section.
Further, as an application of the above adaptive equalization apparatus, there has been proposed a blind equalization apparatus. The blind equalization apparatus is constructed of an adaptive equalization apparatus that does not require a CIR initial value estimated from the unique word.
A representative operation of the blind equalization apparatus will be explained next with reference to the drawings. FIG. 9 is a diagram which shows a structure of a conventional blind equalization apparatus. The reference number 400 denotes a timing adjuster, 401 denotes a timing detector, 402 denotes an adaptive equalizer, and 405 denotes a CIR fixed initial value storage.
According to the conventional blind equalization apparatus shown in FIG. 9, the adaptive equalizer 402 does not require a CIR initial value estimated from a unique word. Therefore, usually, an optional CIR fixed initial value is given from the CIR fixed initial value storage 405 as the CIR initial value. Next, the adaptive equalizer 402 estimates a CIR initial value by using this CIR fixed initial value and a preamble section. Thereafter, the adaptive equalizer 402 sequentially estimates CIRs in the unique word section and the data section, and obtains decision value by using the CIR and the received signal. The timing detector 401 that has received the decision value obtains a decision value timing from a unique word position in the decision value. Last, the timing adjuster 400 adjusts the timing based on the decision value timing.
FIG. 10 is a diagram which shows a communication system that comprises a receiver having the above blind equalization apparatus, and a base station. The reference number 420 denotes a base station, and 421 denotes a mobile station. The reference number 145 denotes a CIR fixed initial value that the adaptive equalizer 402 receives. The direct wave component and the delayed wave component are zero. The adaptive equalizer 402 estimates an initial value of the CIR by using the CIR initial value 145 and the preamble section, and obtain a set CIR 143 in the unique word section. Then, the adaptive equalizer 402 carries out a demodulation in the data section, and outputs a decision value.
However, according to the above conventional adaptive equalization apparatus and blind equalization apparatus, there has been a problem that a timing deviation occurs due to a timing slip. A timing slip that causes this problem will be explained in detail next.
FIG. 11 is a diagram that explains a timing slip in the above conventional adaptive equalization apparatus. The reference numbers 330 and 331 denote channel CIRs, and 340, 341, 350 and 351 denote set CIRs. Assume that a direct wave 123 to a mobile station 321 is interfered by an obstacle, and only a reflection wave 124 reaches the position (A).
In this instance, the adaptive equalizer 302 estimates a transmission CIR from a unique word. However, in the status of the current condition, that is, only one wave (a reflection wave, in this example) arrives, it is not possible to judge whether this wave is a preceding wave or a delayed wave. This status is called a timing ambiguity.
When a wave has been decided as a delayed wave, the mobile station 321 moves from the position (A) to the position (B), and a normal operation is carried out (as usual) when a direct wave arrives.
On the other hand, when a wave has been decided as a preceding wave, the reflection wave is handled as a preceding wave. When a direct wave has arrived without an obstacle, for example, a preceding wave is generated after one preceding wave. In this instance, when there is an obstacle, the adaptive equalizer 302 outputs a decision value after demodulation at a timing obtained from a reflection wave. When there is no obstacle, the adaptive equalizer 302 outputs a decision value after demodulation at a timing obtained from a direct wave. In other words, there occurs a phenomenon that output timings of decision values are different between when an obstacle is present and when an obstacle is not present. This phenomenon is called a timing slip.
FIG. 12 is a diagram that explains a timing slip in the above conventional blind equalization apparatus. The reference numbers 430 and 431 denote channel CIRs, and 440, 441, 450 and 451 denote set CIRs. The reference number 460 denotes a CIR initial value that the adaptive equalizer receives. A direct wave component and a delayed wave component are zero.
In this instance, there is a possibility that the adaptive equalizer 402 estimates CIRs in two ways of ICR 440 and CIR 450 by using a preamble section in a similar manner to the above. However, when the adaptive equalizer 402 has estimated the CIR 450, a timing slip occurs at a point of time when a direct wave has arrived like in the above example.